Optical filtering device and method

ABSTRACT

A wavelength selective filter device is presented suitable for use as a part of a laser cavity for processing light output of a gain section of the laser cavity. The filter structure comprises a resonator structure including at least one closed-loop resonator; and defines an optical coupler structure for coupling light from an input/output of the gain section to propagate through said resonator structure, and a light reflector structure for reflecting light filtered by said resonator structure to propagate through said resonator structure to said input/output of the gain section. The filter structure is configured so as to define two optical paths of substantially the same lengths for light propagation in the resonator structure from and to the coupler structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of International PatentApplication PCT/IL2003/000813 filed Oct. 9, 2003 entitled “OpticalFiltering Device and Method” which claims priority from Israel PatentApplication S/N 152195 filed Oct. 9, 2002, the contents of both of whichare incorporated herein by reference.

FIELD OF THE INVENTION

This invention is generally in the field of optical devices, and relatesto a tunable filter structure based on waveguides and micro-resonators,and a laser device utilizing the same.

BACKGROUND OF THE INVENTION

The demand for increased bandwidth in fiberoptic telecommunications hasdriven the development of semiconductor transmitter lasers usable forpropagation multiple data streams concurrently in a single opticalfiber. In addition to telecommunication applications, semiconductorlasers are now commonly used within audio, visual and personal computersystems where they are employed in the read and, where applicable, writeheads of CD, CDROM and DVD units.

Usually, semiconductor lasers operate with a wavelength spectrumconsisting of a group of several closely spaced wavelengths. However,many applications require the single wavelength operation, narrowlinewidth characteristics. This can be achieved by using a wavelengthselective filter. One example of a commonly used wavelength selectivefilter is an etched grating incorporated within a structure to formDistributed Bragg Reflector (DBR) and Distributed Feedback (DFB) laser.However, statistical variation associated with the manufacture of anindividual DBR and DFB laser results in distribution of the center ofthe fixed wavelength. To solve this problem, the DBR and DFB lasers areaugmented by external reference etalons and usually require feedbackcontrol loops.

Conventional single frequency lasers may also be made to be tunable overwavelength ranges from one nanometer to several tens of nanometers. Inoptical networks, tunable lasers offer many compelling advantages overfixed wavelength devices. This is due to the fact that tunable laserssimplify planning, reduce inventories, allow dynamic wavelengthprovisioning, and simplify network control software, making the tunablelasers suitable for use in wavelength-agile applications, for examplefor wavelength sparing in wavelength division multiplex (WDM) systems.

A tunable laser can be realized as monolithic or hybrid integration. Inthe monolithic integration, all the components are implemented in asemiconductor substrate. In the hybrid integration, a laser cavity isimplemented in a semiconductor chip, while spectrally sensitive elements(external wavelength selective filters) are implemented in a differentoptical medium.

A typical tunable laser is composed of an optical cavity thatencompasses a gain section, and a tunable wavelength selective filter.The gain section includes a medium which can, for example, be asemiconductor based structure utilizing an active semiconductormaterial, e.g. a composition which is selected from InP, InGaAsP, GaAs,InGaAs, AlGaAs, InAlGaAs. In turn, the tunable wavelength selectivefilter can be realized as a micro-resonator, waveguide grating, fibergrating or bulk grating. These features are described, for example, inthe following publications: B. Pezeshki, “Optics & Photonics News,” May2001, p. 34–38; WO 00/24095; WO 00/49689; WO 00/76039; and WO 02/31933.In particular, when a laser utilizes a wavelength selective filter inthe form of a grating, the laser tuning can be performed by themodification of the grating, for example, by applying an external field(such as heat, stress, etc.) or by means of free careers injection(electron plasma).

Micro-ring resonators can provide high quality tunable wavelengthselective filters. A laser constructed in a ring structure is disclosedin WO02/21650, assigned to the assignee of the present application. Sucha laser is unidirectional in its operation. This is associated with theunidirectional nature of a ring resonator: light coupled into the ringat a coupling region propagates in the ring only in one direction. Theuse of ring resonators in a laser thus presents a problem in designingthe laser cavity.

Light coupling into a ring resonator to provide light circulation inopposite directions around the ring has been proposed (e.g., U.S. Pat.No. 5,420,684) for creating a resonant interferometer. According to thistechnique, a passive resonator gyroscope is provided, in which lightfrom a coherent or broadband source is injected into a waveguide beamsplitter is coupled to a fiber optic ring. Frequency modulated light ofsubstantially equal intensity circulates in opposite directions aroundthe ring, and the returning light beams are recombined into the originalwaveguide with a portion of the recombined light provided to aphotodetector. Due to the rotation of the ring, a Sagnac frequency shiftis produced.

SUMMARY OF THE INVENTION

There is a need in the art for, and it would be useful to have, a noveltunable laser device enabling effectively suppressing transmission peaksat wavelengths other than a selected wavelength.

The present invention satisfies the aforementioned need by providing anovel laser cavity based on hybrid integration between a semiconductorgain medium and a tunable filter structure based on waveguides and oneor more micro-resonators (closed-loop resonator).

According to one aspect of the present invention, there is provided awavelength selective filter device which comprises a resonator structureincluding at least one closed-loop resonator; and defines an opticalcoupler structure for coupling light from an input/output of a gainsection to propagate through said resonator structure, and a lightreflector structure for reflecting light filtered by said resonatorstructure to propagate through said resonator structure to saidinput/output of the gain section, the filter structure being configuredso as to define two optical paths of substantially the same lengths forlight propagation in the resonator structure from and to the couplerstructure.

According to one embodiment of the invention, the optical couplerstructure comprises a coupling region between an input/output waveguideconnected to the input/output of the gain section and first and secondwaveguides. The optical coupler thus enables splitting of input lightpropagating in the input/output waveguide from the gain section intofirst and second light portions of substantially equal power and directthese light portions to propagate along two spatially separated paths inthe first and second waveguides, respectively, and enables combining oflight coming from these two paths to propagate through the input/outputwaveguide to the gain section.

According to one possible implementation of this embodiment, the lightreflector structure may be formed by the resonator structureaccommodated between the first and second waveguides and opticallycoupled thereto by first and second spaced-apart coupling regions,respectively. The first and second split light portions thus enter theresonator structure in opposite directions, respectively. Each of thefirst and second coupling regions are spaced from the coupling region ofthe optical coupler substantially the same distance, thereby enablingcombining of first and second light portions propagating through theresonator structure upon reaching said coupling region.

The resonator structure may comprise the single closed-loop resonator,or any number of closed-loop resonators accommodated in a cascade-likefashion between the first and second waveguides.

The resonator structure may comprise first and second resonator unitsoptically coupled to each other via a light combining waveguidestructure. The latter is configured and oriented with respect to theresonator units so as to allow light passage from one of these resonatorunits into the other, such that the light coupled from one resonatorunit into the other propagates in the other resonator unit in the samedirection as it propagated in the first resonator unit.

The light combining waveguide structure may comprise an open-endwaveguide having first and second substantially linear sections withincoupling regions associated with the first and second resonator units,respectively, and a curved section. Alternatively, the light combiningwaveguide structure may comprise a resonator structure including atleast one closed-loop resonator. Each of the resonator units maycomprise the single closed-loop resonator, or at least two closed-loopresonators arranged in a cascaded fashion between the respective one ofthe first and second waveguides and the combining waveguide structure.

According to another possible implementation of this embodiment, thefirst and second waveguides are first and second closed-loop resonatorsof first and second resonator units of the resonator structure. Thefirst and second closed-loop resonators thus share a common couplingregion with the input/output waveguide.

The reflector structure may be formed by the resonator structure and alight combining waveguide structure arranged so as to define first andsecond coupling regions to, respectively, the first and second resonatorunits. The first and second coupling regions may be associated with thefirst and second closed-loop resonators, respectively. Alternatively,each of the first and second resonator units comprises at least twoclosed-loop resonators arranged in a cascade-like fashion between thecommon coupling region and the respective one of said first and secondcoupling regions. The light combining waveguide structure may comprisean open-end waveguide having substantially linear first and secondsections along the first and second coupling regions, and a curvedsection, or may comprise a resonator structure including at least oneclosed-loop resonator.

According to another embodiment of the invention, at least oneclosed-loop resonator of the resonator structure is accommodated betweenand optically coupled to an input/output waveguide connected to theinput/output of the gain section and an additional waveguide. In thiscase, the optical coupler structure is formed by a coupling regionbetween the resonator structure and the input/output waveguide. Thereflector structure is formed by the resonator structure and areflective surface in a path of light propagating through the additionalwaveguide.

According to another broad aspect of the present invention, there isprovided a laser cavity comprising a gain section and a bi-directionalwavelength selective filter device which is optically coupled toinput/output of the gain section via an input/output waveguide forfiltering light coming from the gain section via said input/outputwaveguide and returning filtered light into the gain section via saidinput/output waveguide, wherein said filter device comprises: aresonator structure including at least one closed-loop resonator; anddefines an optical coupler structure for coupling light from theinput/output waveguide to propagate through said resonator structure,and a light reflector structure for reflecting light filtered by saidresonator structure to propagate through said resonator structure tosaid input/output waveguide, the filter device being configured so as todefine two optical paths of substantially the same lengths for lightpropagation in the resonator structure from and to the couplerstructure.

The gain section is preferably formed with an optimized coating on itsone external facet and an anti-reflection coating on its anotherexternal facet to which the input/output waveguide of the selectivefilter device is coupled.

According to yet another broad aspect of the present invention, there isprovided a method for processing a light output of a gain section in alaser device, the method comprising:

-   -   (i) coupling the light output to a wavelength selective filter        structure comprising at least one closed-loop resonator, so as        to select, from said gain section output, light of a        predetermined wavelength band corresponding to the resonance        condition of said filter structure;    -   (ii) directing said selected light to pass through said filter        structure in opposite directions so as to return back into said        gain section.

More specifically, the filter device of the present invention is usefulfor processing output light of a gain section, and is thereforedescribed below with respect to this specific application. The filtersection can, for example, be composed of a semiconductor based structureutilizing Silicon based or other semiconductor material (InP, InGaAsP,GaAs, InGaAs, AlGaAs, InAlGaAs), which can be used in optoelectronics.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting examples only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic block diagram of a laser cavity according to thepresent invention formed by a gain section and a bi-directionalwavelength selective filter structure;

FIG. 2 illustrates a bi-directional wavelength selective filterstructure according to one embodiment of the invention, where the filterstructure utilizes the single ring-like resonator;

FIGS. 3A and 3B illustrate two examples of a bi-directional wavelengthselective filter structure according to another embodiment of theinvention, where the filter structure utilizes two resonator unitscoupled to each other by a curved waveguide;

FIG. 4 illustrates a bi-directional wavelength selective filterstructure according to yet another embodiment of the invention, wherethe filter structure utilizes two ring-like resonators sharing a commonwaveguide section and coupled to each other by an additional curvedwaveguide;

FIG. 5 illustrates a bi-directional wavelength selective filterstructure according to yet another embodiment of the invention, wherethe filter structure utilizes two ring-like resonators sharing a commonwaveguide section and coupled to each other by an additional resonator;and

FIG. 6 illustrates a bi-directional wavelength selective filterstructure according to yet another embodiment of the invention, wherethe filter structure utilizes a multiple ring resonator structure and areflection element.

DETAILED DESCRIPTION OF THE INVENTION

The principles and operation of the bi-directional wavelength selectivefilter structure according to the present invention and a laser deviceutilizing the same may be better understood with reference to thedrawings and the accompanying description, it being understood thatthese drawings and examples in the description are given forillustrative purposes only and are not meant to be limiting. The samereference numerals will be utilized for identifying those components,which are common in all the examples shown in the drawings throughoutthe present description of the invention. It should be noted that theblocks in the drawings are intended as functional entities only, suchthat the functional relationships between the entities are shown, ratherthan any physical connections and/or physical relationships.

Referring to FIG. 1, there is illustrated, by way of a block diagram, atunable external cavity laser device 200 according to the invention. Thelaser device 200 includes a laser cavity that is formed by a gainsection 211 and a bi-directional wavelength selective filter structure100 coupled to the gain section via an input/output waveguide 101. Thelaser cavity is typically associated with a pumping means (which are notspecifically shown) that may be of any known suitable type, such aselectrical pumping as for diode lasers, optical pumping for solid stateor diode lasers.

The construction and operation of the gain section 211 do not form partof the invention, and therefore need not be specifically described,except to note the following. The gain section 211 is preferablyassociated with an optimized coating 213 placed on the external facet ofthe gain section, and an anti-reflection coating 215 on another externalfacet of the gain section to which the input/output waveguide 101 iscoupled. The gain section 211 is constituted of a gain medium that can,for example, be a semiconductor based structure utilizing an activesemiconductor material, e.g., a composition selected from the following:InP, InGaAsP, GaAs, InGaAs, AlGaAs, InAlGaAs. The optimized coating 213is arranged for providing a feedback into the gain section 211. Somespecific but non-limiting examples of such an optimized coating includea quarter-wave coating or a multi-layer coating. The anti-reflectioncoating 215 permits low loss coupling and low back reflections of thelight emanated from the gain section into the input/output waveguide 101and may utilize a quarter-wave coating, or multi-layer coating. Inaddition to the anti-reflection coating, to achieve the requiredreflection reduction, the waveguide 101 coupled to the gain section maybe oriented at an angle with respect to the facet of the gain section,which would result in back reflections not being coupled back into thewaveguide 101.

The filter structure 100 is associated with a tuning means TM operableto affect the resonator frequency band of the filter structure. Such atuning means may utilize one of the following mechanisms: mechanical,electro optic, thermo optic, free carriers injection, or piezoelectric.Output of the gain section 211 collected in the waveguide 101 enters thefilter structure 100, and filtered selected frequency band output fromthe filter structure is collected back at the same waveguide 101 to bereturned to the gain section 211.

The wavelength selective filter structure according to the presentinvention, suitable to be used in a laser device, is a light propagatingstructure, including one or more ring-like resonators and waveguidescoupled thereto, enabling bi-directional propagation of light in thefilter structure so as to enable all the filtered light output from thefilter structure to input the gain section of the laser device.

FIGS. 2 to 6 illustrate various examples of the filter structureaccording to the invention.

FIG. 2 illustrates a wavelength selective filter structure 100 includingan optical coupler 201, and a reflector structure 202. In the presentexample, the optical coupler 201 is implemented as a Y-coupler formed bya coupling region R between the input/output waveguide 101 and twowaveguides 102 and 103. It should, however, be noted that such aY-coupler actually constitutes any suitable coupler providing an equalpower splitting/combining of light between the two waveguides (3 dBcoupler).

The coupler 201 is an optical splitter/combiner adapted for splittinginput light L_(in), coming from the gain section via the input/outputwaveguide 101, into two light portions L₁ and L₂ of substantially thesame power, and directing these light portions towards coupling regionsR₁ and R₂ associated with waveguides 102 and 103, respectively, and isadapted for combining two light components L⁽¹⁾ _(λi) and L⁽²⁾ _(λi)coming from the waveguides 102 and 103, respectively, into an outputlight beam L_(out) to propagate through the waveguide 101 to the gainsection.

The reflector structure 202 is formed by a singe closed-loop resonator(ring) 203 accommodated between the waveguides 102 and 103 so as to beoptically coupled to the waveguides 102 and 103 at two spaced-apartcoupling regions R₁ and R₂ that are spaced from the coupling region R bysegments S₁ and S₂ of the waveguides 102 and 103, respectively. Thearrangement is such that the ring resonator 203 and segments S₁ and S₂of the waveguides 102 and 103 between coupling regions R₁ and R₂ andcoupling region R define equal optical path lengths for light componentsL⁽¹⁾ _(λi) and L⁽²⁾ _(λi).

The device 100 operates in the following manner. Input light L_(in)propagated in the input waveguide 101 reaches the coupler 201 (regionR), and is split equally between the waveguides 102 and 103. At thecoupling region R₁, a wavelength component L⁽¹⁾ _(λi) of the lightportion L₁ having the frequency band coinciding with the resonant bandof the ring resonator 203 is coupled into the ring resonator 203 forclockwise propagation around the ring, while all other wavelengthcomponents of the light portion L₁ continue propagation into a segment104 of the waveguide 102. At the coupling region R₂, a wavelengthcomponent L⁽²⁾ _(λi) of the light portion L₂ having the frequency bandcoinciding with the resonant band of the ring resonator 203 is coupledinto the ring resonator for counterclockwise propagation around thering, while all other wavelength components of the light portion L₂continue propagation into a segment 105 of the waveguide 103. Uponcirculation in the ring resonator, the wavelength components L⁽¹⁾ _(λi)and L⁽²⁾ _(λi) are coupled from the ring resonator 203 into thewaveguides 103 and 102 at the coupling regions R₂ and R₁, respectively,and propagate through the segment S₂ and S₁ of these waveguides to theoptical coupler (splitter/combiner) 201 to pass through the waveguide101 towards the gain section of the laser cavity.

It should be appreciated by a person versed in the art that with thestationary mounted bi-directional wavelength selective filter structure100 and equal waveguide segments S₁ and S₂, the optical path lengths forthe light components propagating in the resonator structure in theopposite directions are equal. Therefore, both light components L⁽¹⁾_(λi) and L⁽²⁾ _(λi) arrive at the coupler 201 (region R) with the sameoptical phase, which results in a coherent buildup of the output lightL_(out) back at the waveguide 101. Therefore, the filter structure 100can effectively suppress transmission peaks at wavelengths other than aselected wavelength (defined by the resonance condition of theresonator). It should be understood that if the light components L⁽¹⁾_(λi) and L⁽²⁾ _(λi) would have been out of phase (e.g., as a result ofmovement of the resonator ring), then a light portions from one of thewaveguides 102 and 103 arriving at the coupler 201 would not be combinedwith the other light component to propagate into the waveguide 101 butwould be directed back into the respective one of these waveguides.

FIG. 3A illustrates a bi-directional wavelength selective filterstructure 110 including a multiple-ring resonator. The structure 110includes an optical coupler 201 formed a coupling region R between theinput/output waveguide 101 and waveguides 102 and 103 (Y-coupler), and aresonator-based reflector structure 202. In the present example, thereflector structure 202 is composed of two resonator units 203A and 203Bcentered to the same selected wavelength band, which are opticallycoupled to the waveguides 102 and 103 via coupling regions R₁ and R₂,respectively, and are optically coupled to each other via an additionalwaveguide 106 at coupling regions R′₁ and R′₂. The waveguide 106presents a light combining waveguide structure that leads the light fromthe first resonator unit into the second one and vice versa. Thiscombining waveguide 106 is oriented with respect to the lightpropagation scheme such that light coupled from the first resonator unit203A into the second resonator unit 203B, propagates in the secondresonator unit in the same direction as it propagated in the firstresonator unit. As shown, the waveguide 106 is configured so as todefine substantially linear segments thereof within coupling R′₁ and R′₂regions, and a curved region. Similarly to the previous example, thering resonators 203A and 203B are accommodated such that segments S₁ andS₂ of the waveguides 102 and 103 between coupling regions R₁ and R₂ andcoupling region R define equal optical path lengths.

The device 110 operates in the following manner. Input light Lpropagated in the waveguide 101 reaches the coupler 201 (region R), andis split into equal light portions L₁ and L₂ directed into thewaveguides 102 and 103. A light component L⁽¹⁾ _(λi) of the lightportion L₁ having the frequency band coinciding with the resonant bandof the ring resonator 203A is coupled into the ring resonator 203A forclockwise propagation around the ring, while all other wavelengthcomponents of the light portion L₁ propagate to the waveguide segment104 of the waveguide 102. This light component L⁽¹⁾ _(λi) is thensubsequently coupled from the resonator ring 203A into the waveguide106, from the waveguide 106 into the second resonator ring 203B forclockwise propagation around the ring 203B, and into the waveguidesegment S₂ through which the twice-filtered thereby light component isdirected back to the coupler 201. The two-ring structure thus carriesout a double-stage filtering of the selected frequency band. The similarlight propagation occurs in the opposite direction: A light componentL⁽²⁾ _(λi) of the light portion L₂ having the frequencies coincidingwith the resonant frequencies of the ring 203B is coupled into the ringresonator 203B for counterclockwise propagation around the ring 203, andis then subsequently coupled into the waveguide 106, ring 203A, andwaveguide segment S₁ to return to the coupler 201. Therefore, both lightcomponents L⁽¹⁾ _(λi) and L⁽²⁾ _(λi) arrive at the coupler 201 withequal optical phase, which results at a coherent buildup of the lightL_(out) back at the input waveguide 101.

In the example of FIG. 3A, each of the resonator units 203A and 203Bincludes a single closed-loop resonator (ring). FIG. 3B exemplifies amultiple-resonator based reflector structure designed similar to that ofFIG. 3A, but having each resonator unit formed of a pair of ring-likeresonators arranged in a cascaded fashion between the respective one ofthe waveguides 102 and 103 and the combining waveguide structure 106.Accordingly, the combining waveguides 106 in the examples of FIGS. 3Aand 3B are oriented identically symmetrical. It should be understoodthat, generally, each of the resonator units may include one or morering resonators and the orientation of the waveguide 106 depends on thenumber of rings in the resonator unit, to thereby ensure the desiredlight propagation directions in the resonator structure as describedabove. For example, for three-ring cascade, the waveguide 106 would bearranged similar to that of FIG. 3A.

FIG. 4 illustrates a bi-directional wavelength selective filterstructure 120 including an optical coupler 201 formed by a commoncoupling region R between the input/output waveguide 101 and twoclose-loop resonator units 203A and 203B; and a resonator-basedreflector structure 202 formed by these resonator units 203A and 203Band a curved waveguide 106 (light combining waveguide structure) coupledto the resonator units via coupling regions R′₁ and R′₂, respectively.Thus, in this example, in distinction to the previously describedexamples, the splitting of the input light and combining of filteredlight is obtained by using two resonator units sharing a commonwaveguide section (coupling region) R. Similarly to the examples ofFIGS. 3A and 3B, each of the resonator units may include more than onering-like resonator, and the orientation of the curved region of thewaveguide structure 106 with respect to the resonators depends on thenumber of resonators in each resonator unit.

The device 120 operates as follows. Input light L_(in) in the waveguide101 reaches the coupler 201 (region R), and is split into equal lightcomponents L⁽¹⁾ _(λi) and L⁽²⁾ _(λi), each having the frequency bandcoinciding with the resonant band of the ring resonator, directed intothe rings 203A and 203B for counterclockwise and clockwise propagationtherein, respectively. These light components L⁽¹⁾ _(λi) and L⁽²⁾ _(λi)are then coupled to the waveguide 106 at the coupling regions R′₁ andR′₂, respectively. Hence, light component L⁽¹⁾ _(λi) propagates in thewaveguide 106 from region R′₁ to region R′₂, where it is coupled intoring 203B. Light component L⁽²⁾ _(λ1) propagates in the waveguide 106from region R′₂ to region R′₁ where it is coupled into ring 203A. Thetwo light components L⁽¹⁾ _(λi) and L⁽²⁾ _(λi) thus reach the coupler201 (region R) with equal optical phase, and are combined into outputlight L_(out) that returns back to the input waveguide 101.

FIG. 5 illustrates a bi-directional wavelength selective filterstructure 130 where, similar to the example of FIG. 4, thesplitting/combining of input/output light is obtained by using tworesonator units sharing a common waveguide section, but in distinctionto the example of FIG. 4, the combining open-end waveguide is replacedby an additional resonator unit (closed-loop waveguide).

The device 130 thus includes an optical coupler 201 formed by a couplingregion R between the input/output waveguide 101 and two closed-loopresonator units 203A and 203B; and a resonator-based reflector structure202 formed by these resonator units 203A and 203B and an additionalresonator unit 106 (constituting a light combining waveguide structure)coupled to the resonator units via coupling regions R′₁ and R′₂,respectively. Input light L_(in) in the waveguide 101 reaches thecoupler 201 (region R), and is split into equal light components L⁽¹⁾_(λi) and L⁽²⁾ _(λi), each having the frequency band coinciding with theresonant band of the respective ring resonator, that are coupled intothe rings 203A and 203B for clockwise and counterclockwise propagationtherein, respectively. Light components L⁽¹⁾ _(λi) and L⁽²⁾ _(λi), uponreaching the coupling regions R′₁ and R′₂, respectively, are coupled tothe resonator unit 106, where they propagate in the opposite directionssuch that these light components L⁽¹⁾ _(λi) and L⁽²⁾ _(λi) are furthercoupled from the ring 106 to rings 203B and 203A, respectively, to becombined at region R into output light L_(out) returning back intowaveguide 101.

Although the examples of FIGS. 4 and 5 show single-ring resonator units203A and 203B, as well as a light waveguide combining structure formedof the single-resonator unit 203 in FIG. 5, it should be understood thatthe same can be implemented by using multiple-resonator units with anappropriate number of resonators to ensure the desired light propagationdirections in the resonator structure, namely that light, coupled fromresonator unit 203A to resonator unit 203B propagates in resonator unit203B in the same direction as that of its original propagation in unit203A.

In the previously described examples, coupling of light between the gainsection and the filter structure was implemented by splitting the lightinto two light portions, and filtering these light portions andreflecting the filtered light back to the input/output waveguide whilepropagating the split light portions via two symmetrical paths definedby resonator-based reflector. The following is an example where thelight coupling does not utilize splitting the input light into spatiallyseparated symmetrical paths, but utilizes the filtering of the inputlight with a resonator-based structure and propagating of the filteredlight towards and away from a light reflector, and filtering again.

FIG. 6 a bi-directional wavelength selective filter structure 140including an optical coupler 201 formed by a coupling region R betweenthe input/output waveguide 101 and a resonator structure 203; and areflector structure 202 formed by this resonator structure 203 and areflector element 208 coupled to the resonator structure. The resonatorstructure is generally a structure formed by at least one ring-likeresonator accommodated between and optically coupled to two waveguides.In this specific but non-limiting example of FIG. 6, the resonatorstructure 203 is composed of two rings 203A and 203B coupled to eachother by an intermediate waveguide 106A via coupling regions R₁ and R₂,coupled to the input/output waveguide 101 via coupling region R betweenthe waveguide 101 and ring 203A, and coupled to the reflector element208 by a waveguide 106B extending between the reflector 208 and acoupling region R′ between the ring 203B and waveguide 106B. It shouldbe understood that the same can be implemented by using a single-stageresonator (with no intermediate waveguide 106A), namely single ringbetween waveguides 101 and 106B or a compound resonator formed by atleast two spaced-apart rings between waveguides 101 and 106B asdisclosed in WO 01/27692 assigned to the assignee of the presentapplication; as well as a multi-stage resonator formed by two or morerings arranged in a cascaded fashion between waveguides 101 and 106Aand/or between waveguides 101 and 106B, provided the reflector element208 is appropriately coupled to waveguide 106A (or waveguide 106B)depending on the design of the resonator structure and the number ofrings therein.

As shown in FIG. 6, input light L_(in) propagates through the waveguide101 and upon reaching the coupler 201 (region R), undergoes wavelengthselective filtering such that a light components L_(λi) having thefrequency band coinciding with the resonant band of the resonatorstructure 203, is coupled to this structure and directed into the ring203A for clockwise propagation therein, while all the other wavelengthcomponents of input light continue propagation through the waveguide101. The filtered light component L_(λi) is then coupled from ring 203Ato ring 203B via waveguide 106A, and then coupled from ring 203B towaveguide 106B to propagate to the reflector 208. The reflected part ofthis light component L′_(λi) returns back through waveguide 106B, iscoupled to ring 203B and then to ring 203A, and returns to the waveguide101 in the direction opposite to that of the input light.

Tuning of the bi-directional wavelength selective filter structure to aselected wavelength can be achieved in a conventional manner by changingthe resonant properties of the filter structure. For example,mechanical, electro optic, thermo optic, free carriers injection, orpiezoelectric effects can be used to cause changes in the size orrefractive index of the waveguides (linear and/or ring waveguides)forming the filter structure.

The use of two or more resonator units in the tunable filter structureprovides for enhanced filter characteristics (rejection ratio), extendedfree spectral range (FSR) and tuning range by using the Vernier effect,where each resonator has a different free spectral range and lasingoccurs only at the frequency where all resonators meet. Due to thedifferent free spectral ranges of the resonators, the combined freespectral range of the reflector structure is obtained from their commonleast common divisor. The combined filter response is obtained by amultiplication of the individual ring resonator frequency responses.Hence only when all resonators are aligned at a given frequency, filterdoes provide a throughput signal at that frequency.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore exemplified without departing from its scopedefined in and by the appended claims. For example, the optical couplerutilized in the tunable bi-directional wavelength selective filterstructure of the invention can be terminated by a taper structure toreduce loss at the interface between the elements. It is apparent thatany number of resonator rings can be concatenated in the resonatorstructure to result in an even more enhanced filtering characteristicsand extended FSR and tuning range.

1. A wavelength selective filter device comprising: a light reflectorstructure comprising first and second resonator units; and an opticalcoupler structure coupling light from an input/output of a laserstructure to propagate through said light reflector structure, saidlight reflector structure being operative to reflect light filtered bysaid first and second resonator units so as to propagate to saidinput/output of the laser structure, said light reflector structurebeing configured so as to define two optical paths of substantially thesame lengths for light propagation in said first and second resonatorunits from and to the coupler structure, and wherein said opticalcoupler structure comprises: first and second waveguides; and a couplingregion between an input/output waveguide connected to said input/outputof the laser structure and said first and second waveguides, saidoptical coupler structure being operative to: split input lightpropagating in said input/output waveguide from the laser structure intofirst and second light portions of substantially equal power; directsaid two light portions to propagate along two spatially separated pathsin said first and second waveguides, respectively; and combine lightcoming from said two paths to propagate through said input/outputwaveguide to the laser structure.
 2. The device of claim 1, wherein saidlight reflector structure is formed by said first and second resonatorunits accommodated between said first and second waveguides andoptically coupled thereto by first and second spaced-apart couplingregions, respectively, said first and second split light portionsthereby entering the light reflector structure in opposite directions,respectively, each of said first and second coupling regions beingspaced from said coupling region of the optical coupler substantiallythe same distance.
 3. The device of claim 2, wherein at least one ofsaid first and second resonator units comprises a single closed-loopresonator.
 4. The device of claim 2, wherein said first and secondresonator units accommodated between said first and second waveguidesare accommodated in a cascade-like fashion between said first and secondwaveguides.
 5. The device of claim 2, further comprising a lightcombining waveguide structure configured and oriented with respect tosaid first and second resonator units so as to allow light passage fromsaid first resonator unit into said second resonator unit, said firstand second resonator units accommodated between said first and secondwaveguides being optically coupled to each other via said lightcombining waveguide structure, such that the light coupled from saidfirst resonator unit into said second resonator unit propagates in saidsecond resonator unit in the same direction as it propagated in saidfirst resonator unit.
 6. The device of claim 5, wherein said lightcombining waveguide structure comprises an open-end waveguide havingfirst and second substantially linear sections within coupling regionsassociated with said first and second resonator units, respectively, anda curved section.
 7. The device of claim 5, wherein said light combiningwaveguide structure comprises at least one closed-loop resonator.
 8. Thedevice of claim 5, wherein each of said first and second resonator unitscomprises at least two closed-loop resonators arranged in a cascadedfashion between the respective one of said first and second waveguidesand said light combining waveguide structure.
 9. The device of claim 1,wherein said first and second waveguides are constituted at leastpartially by said first and second resonator units, said first andsecond resonator units sharing a common coupling region with theinput/output waveguide.
 10. The device of claim 9, wherein said lightreflector structure further comprises a light combining waveguidestructure arranged so as to define first and second coupling regions to,respectively, said first and resonator units.
 11. The device of claim10, wherein each of said first and second resonator units comprises atleast two closed-loop resonators arranged in a cascade-like fashionbetween said common coupling region and the respective one of said firstand second coupling regions.
 12. The device of claim 10, wherein saidlight combining waveguide structure comprises an open-end waveguidehaving substantially linear first and second sections along said firstand second coupling regions, respectively, and a curved section.
 13. Thedevice of claim 10, wherein said light combining waveguide structurecomprises at least one closed-loop resonator.
 14. The device of claim 1,wherein said light reflector structure further comprises: an additionalwaveguide; and a reflective surface said first resonator unit comprisinga closed-loop resonator optically coupled to an input/output waveguideconnected to said input/output of the laser structure, said firstresonator unit being optically coupled to said additional waveguide,said second resonator unit being optical coupled to said additionalwaveguide, said reflective surface being accommodated in a path of lightpropagating through said additional waveguide.
 15. A method forprocessing light output of a gain section in a laser device, the methodcomprising: (i) coupling the light output of a gain section to awavelength selective filter structure comprising at least twoclosed-loop resonators, so as to select from said light output light ofa predetermined wavelength band corresponding to the resonance conditionof said filter structure; and (ii) directing said selected light of thepredetermined wavelength band to pass through said filter structure inopposite directions along two optical paths of substantially the samelengths so as to return back into said gain section, wherein saidcoupling is carried out by providing a coupling region between aninput/output waveguide associated with input/output of the gain sectionand said filter structure, and wherein said directing comprises:splitting the light output at said coupling region into first and secondlight portions of substantially equal power; directing them along firstand second spatially separated paths to be coupled to first and secondresonator units, respectively; passing first and second light portionscoupled to said first and second resonator units, respectively, to saidcoupling region along, respectively, two optical paths of substantiallythe same length; and combining said passed first and second lightportions into an output light beam to propagate through the input/outputwaveguide to the gain section.